A diode-laser is usually defined in an epitaxially-grown separate-confinement semiconductor heterostructure. The heterostructure typically includes one or more active or quantum-well layers bounded by undoped waveguide layers. The waveguide layers provide optical confinement of light generated in the active layer or layers when current is passed through the diode. The waveguide and the active layer or layers are usually referred to as the active region of the diode-laser.
The active layer is bound on one side by one or more n-doped cladding electrical confinement layers. The other side is bounded by one or more p-doped cladding layers. The cladding layers provide the p- and n-sides of the diode and serve to confine carriers in the active region. The carriers “fall” into the quantum-well layer or layers where recombination of the carriers generates the laser radiation. Typically, the heterostructure is grown on an n-type single crystal substrate which is much thicker than the heterostructure. This leaves the p-side of the diode uppermost.
A common type of diode-laser is a “gain-guided” type wherein the laser is defined in the heterostructure by an electrical contact or “stripe” on the p-side (uppermost-side) of the diode. Diode-lasers are plural grown on the substrate, which is diced to leave individual lasers defined in what is referred to as a chip. Alternatively the chip dicing can be such that a linear array diode-lasers is left on what is known to practitioners of the art as a diode-laser bar.
A diode-laser can have a very high efficiency, for example, greater than about 50%. However, that still results in a significant amount of heat being generated in addition to the laser-radiation. For this reason, diode-lasers in individual or array (diode-laser bar) form are invariably mounted “p-side down” on a relatively massive heat-sink. Usually, the heat-sink provides the p-side electrical contact with the strip of the diode laser being defined by etching an equivalent “slot” in an insulating layer covering the “chip” or “bar”, and metalizing the slot.
A diode-laser may have a length between about 1.0 millimeters (mm) and 5.0 mm. The active region has a height (in the thickness direction of the layers of the heterostructure) of about 1.0 micrometers (μm). Standard practice is to select a length in this range and increase power by increasing the stripe-width (emitter width) of the laser. This width may be on the order of about 100 μm for a laser delivering about 5 Watts (W) of power.
A significant problem with increasing the laser (stripe) width is that, generally, the wider the stripe, the less is the brightness of an emitted beam in an axis (the slow axis) parallel to the stripe-width direction. An indicator of brightness is the slow-axis divergence of the beam measured in the far field. The wider the divergence, the less the brightness. The divergence is typically a relatively weak function of current. Diode-laser beam brightness is a critical parameter for certain applications such as pumping of fiber lasers where beam-parameter product of the active fiber is limited by the fiber design, and increased diode-laser beam brightness results in higher pumping levels for the fiber laser.
Diode-lasers have achieved what might be defined as a commodity status commercially, with keen price and performance competition among manufacturers. Even a modest difference in brightness, for example about 10%, between diode-lasers having the same nominal output power and comparable cost could cause a buying decision to made in favor of the brighter diode-laser. Not surprisingly, there is a continuing need for brightness improvement in diode-lasers.